CCD (charge coupled device) image sensors are frequently used for reading the original image of a document or succession of documents in copiers and facsimile apparatus. Such image sensors are typically single dimensional and are capable of electrical scanning in a principal (e.g., horizontal) direction to obtain density information of an image in the form of time sequential electrical signals. Scanning in an auxiliary (e.g., vertical) direction is achieved by moving the original image at right angles with respect to the linear sensor array. As a result, a complete pattern of the original two-dimensional image is created in the form of time sequential signals which are capable of being transmitted to another location, stored in memory, or both.
Each photosensitive site on a CCD image sensor is called a pixel (for picture element). The resolution of a sensor increases with the number of pixels it contains. For good quality document reading purposes, an acceptable resolution is of the order of 300 pixels per inch. Single sensor systems capable of reproducing the long horizontal lines contained in large documents are technically difficult to realize and would, even if technically feasible, be quite expensive. For this reason, multiple sensor systems have been devised in which the length of the sensor arrangement is at least equal to the length of an individual line of the image being read and in which portions of the line being read is overlapped slightly by adjacent sensors.
As the width of a document increases, the resulting increase in line length requires more time to be spent in reading each line if the pixel readout rate (the horizontal clock frequency) for each sensor remains the same and each sensor is read out in sequence. For a system using a total of N sensors, the effective line readout rate would be reduced by a factor of N. Copying a complete document in this manner can be a very slow affair because of the resulting low line readout rate. On the other hand, if the pixel readout rate for each sensor is increased too much, problems of charge transfer inefficiency (usually referred to in the art as CTI) will be encountered. To maintain the line readout rate of a single sensor system, the pixel readout rate for each sensor would have to be increased by a factor of N. For a value of N equal to 6, the individual sensor readout rate would need to be 6 times normal and CTI problems would almost certainly be encountered.
One approach which may be taken to increase the line readout rate of a contact array scanner is to provide each sensor with double ended outputs without changing the pixel readout rate. A 6 sensor arrangement would, in other words, have only the standard pixel readout rate of a single sensor system but would have 12 outputs. With this approach, data from each sensor output would be read into a large digital memory known as a frame store and computer techniques would be used to reconstruct the image being scanned. The frame store could be for either one or two document pages, depending upon the amount of processing needed. Computers and frame stores operating at the speeds at which video sensors operate tend to be quite expensive, so this approach tends not to be of universal applicability by itself even though it may avoid CTI problems.
An alternative approach for increasing the line readout rate of a multiple sensor contact array scanner is simply to read out the contents of all sensors at once, leaving the pixel readout rate for each sensor as it was. This approach, too, tends to be quite expensive as it requires use of both a large number of analog to digital converters and a large number of digital memories. A system with 6 double ended sensors would, for example, require 12 analog to digital converters and 24 digital memories. There is an important need, therefore, to be able to speed up line readout rates in multiple sensor contact array scanners without introducing charge transfer inefficiency problems in the sensors but also without the need to use either expensive frame stores or large numbers of expensive analog to digital converters and digital memories.